In communication and other electric systems, a signal at one radio frequency (RF) band often needs to be converted to another intermediate frequency (IF) band or vice verse using circuits called frequency converters or mixers. During this signal frequency conversion process at a mixer, the ratio between output signal power level after the conversion and input signal power level before the conversion is called conversion gain (CG), which measures the efficiency of the conversion process. The operating bandwidth of a mixer is often defined as the frequency bands where conversion gain is maintained at a reasonably high level.
Meanwhile, the output signal of the mixer is distorted and filled with spurious signals because of the non-linearity of the conversion process. One of the key parameters for measuring the linearity of mixer is the third-order intercept point (IP3) of the signal. If this point is measured using the output signal power level as a reference, it is defined as the output third-order intercept point (Output IP3). Similarly, if this point is measured using the input signal power level as reference, it is defined as input third-order intercept point (Input IP3). The difference of the Output IP3 and Input IP3 is the conversion gain of the mixer.
A double balanced mixer is very popular choice for frequency conversion because of its high spur and port-to-port signal leakage suppressions (isolation between ports). However, the minimum output IP3 of passive double balanced mixer within the operating band is usually 3 dB lower than power at local oscillator (LO) port.
Various techniques have been proposed to improve the linearity of the converted signals: U.S. Pat. No. 6,993,312 B1 to Salib, entitled “Double balanced diode mixer with high output third order intercept point”, is one example of double balanced mixer with resonant resistor-inductor-capacitor (RLC) circuit across the balanced output ports of the mixer balun, which provide better linearity and IP3 at a specified narrow frequency band because of the resonant nature of the RLC network. U.S. Pat. No. 7,197,293 B2 to Vice, entitled “Absorbing sum signal energy in a mixer”, proposed to improve linearity by using a capacitor as a filter connecting the input and output ports and by using a resistor as a load to absorb the spurious between the input and output ports. Some linearity improvement can be achieved at the cost of reduced conversion gain, lower isolation between input and output ports and narrower operating bandwidth. Other ideas were also proposed by Rohde and Poddar in their paper of “Reconfigurable And Cost-Effective FET Mixer”, Wireless and Microwave Technology Conference, 2009, pp 1-7, where a tunable capacitor in series with resistor networks were placed between the gate and drain terminals of the non-linear mixing transistor core. In their topology, Gate terminals are driven by in-phase LO signal and Drain terminals are driven by RF and IF ports. Although linearity improved, the conversion gain and port-to-port isolation suffered.
In another approach, U.S. Pat. No. 7,580,693 B2 to Rohde et al. uses a parallel RC network coupled in series of the in-phase gates of a mixing transistor to shape the wave form of the LO input signal with reduced rise and fall times at the mixing transistor gates. This approach will only work for a certain frequency range with parallel RC providing waveform shaping only at certain frequencies. Also, the in-phase driven mixer core sacrifices the balance operation and high isolation as well.
In certain applications, baluns may be used with a double balanced mixer to great advantage. These baluns, which may include a single-ended-to-differential or a single-ended-to-balanced signal converting circuit (balun), have been widely employed in many radio frequency (RF), microwave and millimeter frequency applications. There have been many approaches and topologies proposed in previous works on the designs of baluns to meet various application demands. The Marchand balun, N. Marchand, “Transmission line conversion Transformers”, Electronics, vol. 17, pp. 142-145, 1944, has become one of the most popular balun topologies to provide low-loss and wide-band differential signals. An alternative topology is described in U.S. Pat. No. 6,292,070; and is often referred to as a back-wave balun. Both topologies can be realized using either distributed elements or lumped elements. And in both balun approaches, the balun comprises a first and second pair of coupled transmission line sections for distributed topology or pair of coupled transformer sections for lumped-element topology. The distributed topologies usually offer better bandwidth performance than their corresponding lumped-element solutions but at the cost of large circuit area, which corresponds to higher manufacturing cost. There have been several publications: Gavela, “A small size LTCC balun for wireless applications”, Proceedings of the European Microwave Conference 2004, pp 373-376; and U.S. Pat. No. 6,819,199, on the size reduction using lumped-element versions for the above two balun topologies.
Many forms of Baluns are known in the art. See: Gavela, “A small size LTCC balun for wireless applications”, Proceedings of the European Microwave Conference 2004, pp 373-376; U.S. Pat. No. 6,819,199; Lin, “An Ultra-broadband Doubly Balanced Monolithic Ring Mixers for Ku- to Ka-band Applications”, IEEE Microwave and wireless components letters, Vol. 17, No. 10, October, 2007; Trifunovic, “Review of Printed Marchand and Double Y Baluns: Characteristics and Application”, IEEE Transactions on Microwave Theory and Techniques, Vol. 42, No. 8, August, 1994; Chen, “Novel Broadband Planar Balun Using Multiple Coupled Lines”, Microwave Symposium Digest, 2006, IEEE MTT-S International, pp. 1571-1574, as well as U.S. Pat. No. 6,683,510 B1 to Padilla, U.S. Pat. No. 7,250,828 B2 to Erb, U.S. Pat. No. 7,068,122 B2 to Weng, U.S. Pat. No. 6,275,689 B1 to Gill and U.S. Pat. No. 5,061,910 to Bouny. All of these above references are incorporated by reference herein.
Marchand balun's differential output branches are connected to ground via the second pair of the coupled sections while the back-wave balun's differential outputs are not grounded at the second pair of the coupled section. Therefore, when DC groundings of the differential ports are needed, the Marchand balun approach is preferred, and when non-zero DC biasing is needed for the differential output port, the back-wave balun approach is preferred. In addition, because the fabrication limitations and parasitic effects limit their bandwidth performance, both balun topologies have their own optimum operation frequency bands. Choosing between Marchand and back-wave baluns based on trade-off in DC biasing and bandwidth performance is often made for each specific application and available fabrication process requirements. In addition, the distributed strip-line baluns with tight broadside coupling are often used to improve bandwidth. But those strip-line baluns require multiple metal layers with rigorously controlled three-dimension profiles, which impose greater fabrication difficulties and higher cost for most planar and semiconductor integrate circuit fabrication processes. Single ended-to-balanced circuits (baluns) are bi-directional in concept, i.e., the input can be single ended and be converted to a differential or balanced output or the input can be balanced or differential and the output single-ended.
For certain applications, a hybrid Marchand/back-wave balun provides the desired DC blocking and biasing features that are not achievable using only either a Marchand-type balun or a back-wave-type balun. Exemplary hybrid Marchand/back-wave baluns are disclosed in U.S. Pat. No. 7,880,557, herein incorporated by reference, which is assigned to Hittite Microwave Corporation of Chelmsford, Mass. The hybrid baluns of the '557 patent provide multi-octave bandwidth with balanced amplitude and phase for mixer and other applications, where single-ended-to-differential conversions are critical for overall circuit performance. The hybrid baluns of the '557 patent may be implementable using either distributed coupling lines or lumped elements.